The invention relies on new security techniques derived from iridescent and non-iridescent variable optical devices. More specifically the invention take advantage of some properties of liquid crystals whose usefulness in device security applications has been demonstrated: anisotropy, optical birefringence, the different phases that the liquid crystal can adopt, etc. When these properties are correctly manipulated, several optical effects are generated; these are easily verifiable by naked eye or by using simple optical elements.
Specifically, Rumiko Yamaguchi et al. in “Polarized latent image forming in liquid crystal devices using polymer surface photo-modification technique”, 2004, Proceedings of SPIE Vol. 5618, pp. 166-173, propose to induce two orthogonal orientations on each confinement plate. This generates one black and white image on each plate, depending on the liquid crystal alignment with respect to the plane of the confinement plates and the polarizer orientation or the impinging light polarization.
However the device of Rumiko Yamaguchi et al. shows many drawbacks: the use of combined alignment techniques on the same surface (UV+buffing, UV+heat), the limitation to a maximum of two latent images per device, and the need of rigid external confinement plates that mar their integration in other elements such as documents, packaging or bank notes.
In document EP1894736-A2, Hoffmueller describes a model based on a liquid crystal featuring chiral behavior (liquid crystal molecules show orientational order and optical activity, generating a rotation of the light as it travels through). Two liquid crystal layers are placed on a support substrate, the second one being chiral. Interaction between the chiral layer and the first layer, whose molecules are partially oriented, generates a variable interlacing between layers that produces a color variation by the tilt/twist of the liquid crystal molecules. This technique produces two or more well-delimited regions having different colors. However, it has many drawbacks: no grayscale generation is provided, it employs a double layer of liquid crystals, color differences are only obtained in separate regions and the creation of high-resolution details or recognizable motifs is avoided. Moreover this technique hinders the generation of quadruple latent images (or even double latent images), and the elimination of external confinement plates.
WO2008058670-A1 proposes a device consisting of a liquid crystal cell structure inserted in the document that must be protected. The liquid crystal is a chiral nematic liquid crystal, and a linear light polarizer is attached to the “cell”. The authentication of the verification element is checked with a verification unit including other linear polarizer that incorporates an emblem or any other motif as wished. When the liquid crystal is observed between the polarizers, a dark image of the emblem must be seen. Two important disadvantages of this technique, besides those cited in EP1894736-A2 are the need of including two linear polarizers for verification, and the need of a verification unit associated to one of the said polarizers.
WO2008067932-A3 proposes another device based on liquid crystal alignment (molecular alignment on the surfaces). The device features two alternatively repeated motifs generated by different molecular orientation depending on the zone. Homogeneous orientation is induced on one zone and homeotropic orientation is induced in other zone, so that they are, clearly visible through a linear polarizer. The drawbacks are: the requirement of two kinds of alignments (homogeneous and homeotropic) and a layer of inorganic material (aluminum or aluminum oxide). Moreover, no grayscale is available; color differences are restricted to regions and the creation of high-resolution details or just recognizable motifs is thwarted.
WO2008138539-A1 proposes an alternative for the security device to show complex patterns. The device is made of three superposed polymeric sheets: the first and third sheets are conveniently oriented polymeric liquid crystal; the second sheet is just an intermediate layer between the other sheets. Both oriented liquid crystal sheets give two latent visible images by means of a polarizer. Either image can be seen by rotating 90° the polarizer. Images are binary, i.e., with no grayscale. Main drawback: the manufacturing process is fairly complex (three polymeric sheets, stands . . . ); moreover, only a single latent image can be made visible with a linear polarizer; a 90° rotation of the polarizer undergoes a color inversion of the image.
JP20090 78418-A proposes a structure made of: a reflective layer, an intermediate layer and the liquid crystal layer where the latent image is formed by inducing two different alignments to the liquid crystal molecules according to a predetermined alignment pattern. All the layers are covered by a protective layer whose refractive index is matched with the index of the intermediate layer in the visible region. Main drawback: this structure has a single side and requires a reflective layer.